This invention relates to a rotation transmission device used on a four-wheel drive vehicle and capable of selectively transmitting the driving force for acceleration and cutting off the transmission when brakes are applied.
FIG. 23 shows a conventional rotation transmission device employing a mechanical clutch mounted on the drive train of a four-wheel drive vehicle and is capable of selectively transmitting the driving force to the driven wheels and cutting it off (Japanese Unexamined Patent Publication No. 5-118358).
This device comprises an outer ring 101 as a driven member, an inner member 102 as a driving member rotatably mounted in the outer ring 101, a large-diameter retainer 103 and a small-diameter retainer 104 mounted between the outer ring 101 and the inner member 102, and sprags 105 as engaging elements received in pockets formed in the retainers 103, 104 and adapted to engage the outer ring 101 and the inner member 102 together.
The large-diameter retainer 103 is coupled through pins 106 to the inner member 102 and the small-diameter retainer 104 with gaps left therebetween in the direction of rotation. To a tubular member 110 coupled to the pin 106 is coupled an elastic member 107 for imparting a turning effort to the retainer 103 from one direction.
To the end of the tubular member 110 is coupled a turning effort imparting means 109 for imparting a turning effort to the retainer 103 from the direction opposite to the direction of turning effort applied by the elastic member 107. When the rotational direction of the inner member 102 is reversed, a one-way clutch 108 is adapted to engage or disengage, thus changing over the direction of turning effort applied to the large-diameter retainer 103.
This device B may be mounted alone on the propeller shaft for driving the rear wheel or front wheel as shown in FIGS. 19 and 20 or it may be mounted together with a differential device such as a center differential or a limited slip differential device H as shown in FIG. 21.
In the driving mechanism shown in FIG. 19, while the vehicle is moving forward climbing a slope, the two retainers 103, 104 move relative to each other to the positions shown in FIG. 24, thereby moving the sprags to a position where they are ready to engage. Now let us suppose that the vehicle is brought to a stop while climbing a slope and then begins to move backward down the slope by gravity with the transmission in the neutral position. If a play formed in the rotational direction between the front wheels.fwdarw. front differential E.fwdarw. rotation transmission device B is smaller than the play formed in the rotational direction between the rear wheels.fwdarw. rear differential F.fwdarw. transfer C.fwdarw. rotation transmission device B, the outer ring 101 of the rotation transmission device B, which is coupled to the front wheels, will begin to rotate before does the inner member 102.
In this case, the sprags 105 are held by the large-diameter retainer 103 in the forward-travel ready-to-engage position as shown in FIG. 24. If the outer ring 101 begins to rotate backward in this state, the sprags 105 will engage both the outer ring 101 and inner member 102. Once these members engage one another, it is impossible to change over the position of the large-diameter retainer 103 even if the inner member 102 is rotated thereafter. The front wheels and the rear wheels are thus directly connected together. If the vehicle turns a tight corner in this state, a braking phenomenon will occur at a tight corner.
The driving mechanism shown in FIG. 20 has a similar problem. Namely, if the rotational-direction play between the rear wheels and the transmission device is smaller than the play between the front wheels and the transmission device, it is difficult to smoothly change over the position of the large-diameter retainer. The sprags are thus often left inclined in the direction opposite to the desired direction. If this happens, the inner member 102 and the outer ring 101 are coupled together.